␥-Herpesviruses, Epstein-Barr virus, and Kaposi's sarcoma-associated herpesvirus are important human pathogens, because they are involved in tumor development. Murine ␥-herpesvirus-68 (MHV-68 or ␥HV-68) has emerged as a small animal model system for the study of ␥-herpesvirus pathogenesis and host-virus interactions. To identify the genes required for viral replication in vitro and in vivo, we generated 1,152 mutants using signature-tagged transposon mutagenesis on an infectious bacterial artificial chromosome of MHV-68. Almost every ORF was mutated by random insertion. For each ORF, a mutant with an insertion proximal to the N terminus of each ORF was examined for the ability to grow in fibroblasts. Our results indicate that 41 genes are essential for in vitro growth, whereas 26 are nonessential and 6 attenuated. Replication-competent mutants were pooled to infect mice, which led to the discovery of ORF 54 being important for MHV-68 to replicate in the lung. This genetic analysis of a tumor-associated herpesvirus at the whole genome level validates signature-tagged transposon mutagenesis screening as an effective genetic system to identify important virulent genes in vivo and define interactions with the host immune system. functional mapping ͉ herpesvirus ͉ bacterial artificial chromosome ͉ deoxyuridine-triphosphatase ͉ transposition

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